Many types of electrical loads such as motors require the regulation of the delivery of average power or frequency. Such loads are usually driven by either sine waves or pulse width modulated square waves which are generated from a D.C. supply voltage by means of a conventional bipolar transistor.
Theoretically, to obtain the most efficient use of such a load, a sine wave is utilized. However, excitation with a sine wave requires the transistor driver to operate as a linear device. Unfortunately, when operated in this fashion, the transistor must be capable of dissipating relatively large amounts of power.
An alternative to using sine waves, is the use of square waves. As far as the load is concerned this is slightly less efficient and involves an increase in output variations such as torque in a motor. However, square wave drives provide a significant increase in the efficiency of the driving circuit. This is because very little power is dissipated in the drive transistors when they are switched, very rapidly, either fully ON or full OFF in a binary digital manner. Unfortunately, the use of such a digital driver involves a serious drawback, which is due to the nature of square waves themselves; a square wave has leading and trailing edges with very fast rise and fall times. The result is a nearly instantaneous change in the flow of currents and voltages which leads to the generation of undesirable electromagnetic interference (EMI) as the power is delivered to the load.